iio: documentation: Add ABI documentation for *_mean_raw

Add ABI documentation for in_*_mean_raw files, which are
already supported and used in the kernel for some time.
Signed-off-by: Sebastian Reichel <sre@debian.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -210,6 +210,14 @@ Contact:	linux-iio@vger.kernel.org
 Description:
 		Scaled humidity measurement in milli percent.
 
+What:		/sys/bus/iio/devices/iio:deviceX/in_X_mean_raw
+KernelVersion:	3.5
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Averaged raw measurement from channel X. The number of values
+		used for averaging is device specific. The converting rules for
+		normal raw values also applies to the averaged raw values.
+
 What:		/sys/bus/iio/devices/iio:deviceX/in_accel_offset
 What:		/sys/bus/iio/devices/iio:deviceX/in_accel_x_offset
 What:		/sys/bus/iio/devices/iio:deviceX/in_accel_y_offset